The invention relates to a process for the preparation of a carboxylic acid RCOOH wherein R represents an alkyl, cycloalkyl or aralkyl group, by heating an ester of formic acid HCOOR in the presence of carbon monoxide, a soluble rhodium catalyst and an iodide and/or bromide source. The invention relates in particular to the preparation of acetic acid from methyl formate.
U.K. Pat. Specification No. 1,286,224 and U.S. Pat. No. 4,194,056 describe processes for the preparation of acetic acid wherein methyl formate is heated in the presence of carbon monoxide, a rhodium catalyst and a halogen-containing promoter, particularly methyl iodide. However, rhodium is costly and investigations have been carried out to find processes in which practicable results are obtained by using less expensive catalysts under moderate reaction conditions. Such a process, in which, among other things, a palladium or ruthenium compound is used, is disclosed in German Offenlegungsschrift No. 3,046,899. However, the conversion of methyl formate proceeds at a considerably lower rate than in the presence of rhodium compounds, which is the reason why, generally, the use of the latter compounds remains preferred. On account of the high cost of rhodium, however, it is important that the catalyst system which eventually is formed by interaction of the rhodium compound with carbon monoxide, halogen-containing promoter and other ligands in the reaction mixture, be as active as possible, so that under moderate reaction conditions the reaction will proceed at a rate suitable for use in actual practice.
The rate at which the carboxylic acid is formed can be expressed as the number of gram atoms carboxylic acid that are formed per gram rhodium per gram halogen present in the halogen-containing promoter per hour. This is an adequate method of expression when determining the activity of the catalyst system, since generally the reaction rate is directly proportional to both the quantity of rhodium and the quantity of halogen-containing promoter present in the reaction mixture. Not only is it important for the quantity of carboxylic acid formed per gram rhodium per hour to be as large as possible, but also for this to be achieved in the presence of as little halogen-containing promoter as possible. Not only do halogen compounds, such as for instance methyl iodide, often have corrosive properties, they are also fairly volatile, so that during recovery of the reaction product by distillation they leave the reactor and must be recirculated. For a process to be carried out on a large scale it is naturally of importance that the quantity of material to be recirculated is kept as small as possible.
It has now surprisingly been found that certain compounds of pentavalent phosphorus, arsenic or antimony, which will be defined hereinafter, have a strong promoter activity during the preparation of acetic acid from methyl formate in the presence of a rhodium catalyst and an iodide and/or bromide source.